THERE’S no animal that symbolises rainforest diversity quite as spectacularly
as the tropical butterfly. Anyone lucky enough to see these creatures flitting
between patches of sunlight cannot fail to be impressed by the variety of their
patterns. But why do they display such colourful exuberance? Until recently,
this was almost as pertinent a question as it had been when the 19th-century
naturalists, armed only with butterfly nets and insatiable curiosity, battled
through the rainforests.
These early explorers soon realised that although some of the butterflies’
bright colours are there to attract a mate, others are warning signals. They
send out a message to any predators: “Keep off, we’re poisonous.” And because
wearing certain patterns affords protection, other species copy them. Biologists
use the term “mimicry rings” for these clusters of impostors and their
evolutionary idol.
But here’s the conundrum. “Classical mimicry theory says that only a single
ring should be found in any one area,” explains George Beccaloni of the Natural
History Museum, London. The idea is that in each locality there should be just
the one pattern that best protects its wearers. Predators would quickly learn to
avoid it and eventually all mimetic species in a region should converge upon it.
“The fact that this is patently not the case has been one of the major problems
in mimicry research,” says Beccaloni.
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In pursuit of a solution to the mystery of mimetic exuberance, Beccaloni set
off for one of the megacentres for butterfly diversity, the point where the
western edge of the Amazon basin meets the foothills of the Andes in Ecuador.
“It’s exceptionally rich, but comparatively well collected, so I pretty much
knew what was there,” says Beccaloni. “The trick was to work out how all the
butterflies were organised and how this related to mimicry.”
Working at the Jatun Sacha Biological Research Station on the banks of the
Rio Napo, Beccaloni focused his attention on a group of butterflies called
ithomiines. These distant relatives of Britain’s Camberwell Beauty are abundant
throughout Central and South America and the Caribbean. They are famous for
their bright colours, toxic bodies and complex mimetic relationships. “They can
comprise up to 85 per cent of the individuals in a mimicry ring and their
patterns are mimicked not just by butterflies, but by other insects as diverse
as damselflies and true bugs,” says Philip DeVries of the Milwaukee Public
Museum’s Center for Biodiversity Studies.
Even though all ithomiines are poisonous, it is in their interests to evolve
to look like one another because predators that learn to avoid one species will
also avoid others that resemble it. This is known as Müllerian mimicry.
Mimicry rings may also contain insects that are not toxic, but gain protection
by looking like a model species that is: an adaptation called Batesian
mimicry. So strong is an experienced predator’s avoidance response that even
quite inept resemblance gives some protection. “Often there will be a whole
series of species that mimic, with varying degrees of verisimilitude, a focal or
model species,” says John Turner from the University of Leeds. “The results of
these deceptions are some of the most exquisite examples of evolution known to
science.” In addition to colour, many mimics copy behaviours and even the
flight pattern of their model species.
But why are there so many different mimicry rings? One idea is that species
flying at the same height in the forest canopy evolve to look like one another.
“It had been suggested since the 1970s that mimicry complexes were stratified by
flight height,” says DeVries. The idea is that wing colour patterns are
camouflaged against the different patterns of light and shadow at each level in
the canopy, providing a first line of defence against predators. “[But] the
light patterns and wing patterns don’t match very well,” he says. And
observations show that the insects do not shift in height as the day progresses
and the light patterns change. Worse still, according to DeVries, this theory
doesn’t explain why the model species is flying at that particular height in the
first place.
High flyers
“When I first went out to Ecuador, I didn’t believe the flight height
hypothesis and set out to test it,” says Beccaloni. “A few weeks with the
collecting net convinced me otherwise. They really flew that way.” What he
didn’t accept, however, was the explanation about light patterns. “I thought, if
this idea really is true, and I can work out why, it could help explain why
there are so many different warning patterns in any one place. Then we might
finally understand how they could evolve in such a complex way.”
The job was complicated by the sheer diversity of species involved at Jatun
Sacha. Not only were there 56 ithomiine butterfly species divided among eight
mimicry rings, there were also 69 other insect species, including 34 day-flying
moths and a damselfly, all in a 200-hectare study area. Like many entomologists
before him, Beccaloni used a large bag-like net to capture his prey. This
allowed him to sample the 2.5 metres immediately above the forest floor. Unlike
many previous workers, he kept very precise notes on exactly where he caught his
specimens.
The attention to detail paid off. Beccaloni found that the mimicry rings were
flying at two quite separate altitudes. “Their use of the forest was quite
distinctive,” he recalls. “For example, most members of the clear-winged mimicry
ring would fly close to the forest floor, while the majority of the 12 species
in the tiger-winged ring fly high up.” Each mimicry ring had its own
characteristic flight height.
However, this being practice rather than theory, things were a bit fuzzy.
“They’d spend the majority of their time flying at a certain height. But they’d
also spend a smaller proportion of their time flying at other heights,”
Beccaloni admits. Species weren’t stacked rigidly like passenger jets waiting to
land, but they did appear to have a preferred airspace in the forest. So far, so
good, but he still hadn’t explained what causes the various groups of ithomiines
and their chromatic consorts to fly in formations at these particular
heights.
Then Beccaloni had a bright idea. “I started looking at the distribution of
ithomiine larval food plants within the canopy,” he says. “For each one I’d
record the height to which the host plant grew and the height above the ground
at which the eggs or larvae were found. Once I got them back to the field
station’s lab, it was just a matter of keeping them alive until they pupated and
then hatched into adults which I could identify.”
Potatoes preferred
Beccaloni’s painstaking collection of eggs and caterpillars, combined with
details about what was found eating what and where, eventually enabled him to
identify the host plants on which the larvae of 16 ithomiine species feed. Most
use various species of the potato and tomato family, Solanaceae. And there was a
very distinct pattern: females of the lowest-flying rings laid their eggs on
herbs and small shrubs, while the higher-flying species used small trees. “Such
a precise association was quite unexpected,” he says.
Beccaloni believes that this also happens in the upper reaches of the forest
canopy. “At Jatun Sacha some potato relatives are epiphytic, growing, like
orchids and urn plants, high in the canopy of the forest’s major trees,” he
says. Although the methods he used did not allow him to prove it, he thinks his
butterfly species are stratified into three separate levels in any given area,
with those from the highest flying rings laying their eggs on these epiphytic
shrubs.
For females then, stratification among mimicry rings has little to do with
light and predators, and everything to do with where the larval food plants are.
“Ithomiines turned out to be incredibly choosy,” says Beccaloni. “Females would
seek out just one or two species of plant and no more.” They spend a large
proportion of their lives searching for host plants and so must fly at a height
that maximises their chances of finding the plants they lay their eggs on.
So what about the males? Where they fly depends mostly on the females, but
Beccaloni has discovered that the presence of certain plants also keeps them
flying with their own ring. Males seek out the nectar of Eupatorium, a
common weedy tropical daisy, and the decomposing foliage of species of tropical
borage and lianas—in fact, any plant that contains bitter-tasting
pyrrolizidine alkaloids (PAs). These are the compounds that make ithomiines
poisonous. Males eat the noxious chemicals and transfer them to females with
their sperm during mating. The females then use the PAs to defend themselves and
their eggs. And the fact that plants where females lay their eggs and those
where males load up with toxins don’t occur at precisely the same level in the
forest’s skyscraper helps to explain some of the flight-height fuzziness.
Missing link
“These were very important results,” recalls DeVries. “Beccaloni had
discovered the missing link between the flight height and ecology.” But one
mystery remained. With eight mimicry rings, stacked three storeys high, plant
height alone couldn’t account for all the diversity. What was going on? An
intrigued DeVries decided to go to Jatun Sacha for a closer look.
To his delight, he and his students found that the ithomiines and their
mimics are organised horizontally as well as vertically. And, once again, they
seemed to be clustering around patches of specific plants. “Different vegetation
types had different mimicry rings associated with them. These were often closely
related to species occurring just a few feet away but, bound by their fidelity
to site-specific host plants, they almost never met,” says DeVries. This
explains how several quite similar rings could happily exist in close
proximity.
Some experts, including André Frietas from Campinas State University
in Brazil, believe that the link between mimicry rings and vegetation patches
has been oversimplified and the explanation is almost too neat. “Simple
relations of cause-effect are difficult to detect in such cases,” says Frietas.
“Certainly other variables are operating, like predator pressures, light and
shade mosaic, and variation in the community structure of species involved in
the mimicry rings,” he says. “These were not analysed in Ecuador, but have been
shown to be influential elsewhere in the neotropics. I think much additional
study should be done before we decide we have a good answer to this
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Turner has a further concern. “The proposal only works if predators are
either stratified too or if they retain a long-term memory of exactly where in
the forest they caught a particular butterfly. As yet, there is no evidence for
this. Those bird groups showing strong stratification are not major butterfly
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But others see this work as a big breakthrough. “Considering that mimicry
theory is over 100 years old, there is surprisingly little good fieldwork to
test it,” says Mike Speed of Liverpool’s Hope University, who specialises in
computer modelling of ecological problems. “The work of Beccaloni and DeVries
gives new impetus to the field. It gives all of us working on mimicry a valuable
glimpse of the complex structure of real ecologies.” And the importance of these
findings goes beyond the evolution of colour pattern in butterflies. “It may
give us a mechanism for population divergence that could theoretically lead to
speciation without geographical barriers,” says Speed. “And that’s incredibly
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Further reading:
Association of co-mimic ithomiine butterflies on small spacial and temporal
scales in neotropical rainforest by Phillip DeVries and others, Biological
Journal of the Linnean Society, vol 67, p 73 (1999) -
Vertical stratification of ithomiine butterfly mimicry complexes by George
Beccaloni, Biological Journal of the Linnean Society, vol 62, p 313 (1997)